Modern disc drives typically comprise one or more magnetic discs that are coated with a magnetizable medium and mounted on a hub of a spindle motor for rotation at a constant high speed. Information is written to and read from nominally circular, concentric data tracks on the discs through the use of an actuator assembly, which rotates during a seek operation about a bearing shaft assembly positioned adjacent the discs. The actuator assembly typically includes a plurality of actuator arms that extend over the discs, with one or more flexures extending from each of the actuator arms. Mounted at the distal end of each of the flexures is a transducer, including a write element and a read element, for writing information to, and/or reading information from, the tracks of the disc drive, respectively.
To move the transducer over the appropriate track for reading or writing information, the actuator assembly typically includes a voice coil motor (VCM), which includes a coil attached to the actuator assembly, as well as one or more permanent magnets that establish a magnetic field in which the coil is immersed. The controlled application of current to the coil causes magnetic interaction between the permanent magnets and the coil moves in accordance with the well known Lorentz relationship. As the coil moves, the actuator assembly pivots about the bearing shaft assembly, and the heads are caused to move across the surfaces of the discs.
Each of the concentric data tracks on a disc is typically angularly divided into a plurality of data sectors. In addition, special servo information is typically included in each track to assist in determine the position of the read/write head. The servo information is typically written in a plurality of servo wedges that are angularly spaced from one another and interspersed between data sectors around each track of each disk. Each servo wedge typically includes a track identification (ID) field and a group of servo bursts. A servo control system in the disc drive applies an appropriate current to the coil of the voice coil motor to move the transducer toward a desired track during a coarse “seek” mode using the track ID field as a control input. Once the transducer is generally over the desired track, the servo control system uses the servo bursts to keep the transducer over that track in a fine “track follow” mode. The read element of the transducer generally reads the servo bursts to produce a position error signal PES that is indicative of the position of the read element, relative to a predetermined radial position on the track.
To achieve data integrity and high data transfer rates, it is critical that the read and write elements be respectively maintained as close as practicable over the center of each track during read and write operations. For example, even if data are properly written by the write element in a centered relationship over a selected track, attempting to subsequently read the data while the read element is positioned some distance away from the center of the track may result in an unacceptable number of read errors, due to the inability of the read element to properly detect the written data, as well as the potential interference from the selective magnetization of an adjacent track. More significantly, writing data too far away from the track center can prevent subsequent data recovery when the head is centered over the track, and can also corrupt data stored on the adjacent track. The distance a given element is from the center of the track is commonly referred to as the track misregistration (TMR) of the element. As will be appreciated, in order to achieve superior performance from the disc drive, it is necessary to keep TMR to a minimum.
There are number of causes of TMR in a disc drive, principal of which is what is known as runout error. Runout errors come in two primary forms, repeatable runout (RRO) errors and non-repeatable runout (NRRO) errors. RRO errors are synchronous with disc rotation and may be the result of various effects, such as an error in writing the servo burst information on the disc or a disc shift caused by an eccentricity in the disc. NRRO errors are typically not synchronous with disc rotation and may be caused, for example, by bearing defects, noise, spindle motor imperfections, and servo loop response errors. While in general, RRO errors can be removed or compensated for in the disc drive, NRRO errors generally cannot be removed.
In order to produce disc drives having high track densities, and thus high data storage capacities, it is typical during the design and assembly processes of the disc drive to perform a number of tests on the disc drive and its various elements. For example, during the design and assembly processes, actuator assemblies, or parts of actuator assemblies, are tested on a disc drive spin-stand to determine their operational characteristics. Similarly, entire disc drives are also tested, both in the design process and in the manufacturing and certification process, to determine their operation characteristics.
Several of the parameters that are typically measured on the disc drive and its components during the design and/or manufacturing and certification tests include read track misregistration (RTMR) value, write track misregistration (WTMR) value, write-to-write track misregistration (WWTMR), write-to-read track misregistration (WRTMR) value, bit error rate (BER), and average position error (APE) margin. The RTMR value defines the distance the read element is from the predetermined center of the track during a read operation. The WTMR value defines the distance the write element is from the predetermined center of the track during a write operation. The WWTMR defines the wiggling or distance between two written tracks. The total error, that is the distance between where the write element was relative to the center of the track while writing the data and where the read element was relative to the center of the track while reading the data, is the WRTMR. WRTMR is a very important parameter in disc drive design and manufacture, because it represents the sum of most of the things that can cause data-handling problems inside the disk drive. In general, the WTMR and WRTMR describe the servo-mechanical system performance of the disc drive. The BER is the ratio of the number of defective bits on a track to the total number of bits recorded on the track.
The APE margin refers to the margin, as a percentage of track pitch (width), that the read element can be off track while maintaining a given BER, such as, for example, 1E-5 or 1E-6 errors. In general, the APE margin characterizes the performance of various elements or subsystems of the disc drive. In particular, the APE margin is useful in characterizing the disc drive recording subsystem, including the head(s), preamplifier(s), disc(s) and data channel of the disc drive.
As is known, the APE margin is typically measured by writing predetermined data to a track using the write element and reading the written data from the track with the read element while progressively positioning the read element at predetermined distances from the center of the track. The APE margin is then determined for a given BER. That is, the APE margin is the position of the read head from the track center at which a particular BER is measured. Typically, the APE is measured at BER values of 1E-5 and/or 1E-6.
While measuring the APE margin as just described is relatively accurate in measuring APE margins, there is an inherent problem with this type of measurement that relates to the WRTMR. In particular, the APE margin of a given disc drive will be inversely proportional to the WRTMR of the disc drive. The greater the WRTMR of the disc drive, the lower the APE margin. As such, the typical process used to measure the APE margin of a disc drive will not yield an accurate or ideal APE margin. That is, the typical process used to measure the APE margin will not yield an APE margin at zero WRTMR.
As such, there is a need in the art for systems and processes that can more accurately measure APE margins. More particularly, there is a need in the art for systems and processes that can accurately measure APE margins at zero WRTMR.